1. Field of the Invention
This invention relates to near-infrared transmitting black azo pigments and black azo pigment compositions containing the same. The term xe2x80x9cnear-infraredxe2x80x9d as used herein means light of 800 to 2,000 nm in wavelength.
2. Description of the Related Art
Carbon black, aniline black, iron oxide black and the like have been used as black pigments for many years. These pigments absorb light of wavelengths from the ultraviolet range to the far-infrared range, have no transmitting property for near-infrared light, and have a tendency to absorb infrared rays, i.e., heat waves. Materials colored by these pigments, therefore, tend to become hot by direct sunlight. When these conventional black pigments are used as colorants in paints for electronic parts, they are poor in electrical insulating property.
In recent years, on the other hand, black pigments having properties not available from black pigments commonly used to date, such as carbon black and aniline black, are required in an increasing number of fields owing to developments of lasers, especially semiconductor lasers and sensors therefor. Examples of such fields can include fields making use of such black pigments as colorantsxe2x80x94such as paints for electronic parts, temperature rise preventing paints for automobiles or construction materials, and cheese cloths for agricultural usexe2x80x94in addition to infrared communications, optical filters and illegal copying preventing prints, all of which make use of black pigments.
Carbon black, aniline black and the like are accompanied by drawbacks in that they are poor in electrical insulating property and are not suited for the coloration of electrical parts and the like, although they absorb light of wavelengths from the ultraviolet range to the far-infrared range and have conventionally been used as black pigments in various coloring applications. These properties are inherent to such pigments themselves and cannot be improved. There is, accordingly, an outstanding demand for a black pigment free of such drawbacks, namely, having high transmittance for near-infrared rays.
An object of the present invention is, therefore, to provide a black pigment, which can transmit near-infrared rays at high transmittance and can also be used for coloring electrical parts and the like. Another object of the present invention is to provide a pigment composition making use of the black pigment as a colorant.
The present inventors have proceeded with an extensive investigation to achieve the above-described objects. As a result, it has been found that the transmittance of near-infrared rays is increased by changing the crystalline form of a specific black azo pigment into a thin, long crystalline form or a leaf-shaped crystalline form, leading to the completion of the present invention. The above-described objects of the present invention can be achieved by the present invention which will be described hereinafter. Described specifically, the present invention provides a near-infrared transmitting black azo pigment represented by the following formula (1) and having a crystalline form selected from the group consisting of a thin, long crystalline form and a leaf-shaped crystalline form. 
wherein R represents at least one group selected from the group consisting of lower alkyl groups having 1 to 3 carbon atoms and lower alkoxy groups having 1 to 3 carbon atoms, n stands for an integer of from 1 to 5, and, when n is other than 1, Rs may be the same or different. The present invention also provides a near-infrared transmitting black azo pigment composition, which comprises the above-described near-infrared transmitting black azo pigment and a near-infrared transmitting material as a vehicle.
The near-infrared transmitting black azo pigment according to the present invention is low in near-infrared ray reflectance and high in near-infrared ray transmittance, different from the known black azo pigment having the same chemical structure (the near-infrared ray reflectance and near-infrared ray transmittance of which are high and low, respectively).
The near-infrared transmitting black azo pigment according to the present invention are useful as a colorant in temperature rise preventing paints for automobiles, construction materials and the like, paints for agricultural cheese cloths and paints for electronic parts, and also as a colorant or the like in infrared communications and optical filters and also for the prevention or illegal copying.